This affects only YouTube videos that have been uploaded to the gallery. Behaviour is normal, except that if the mouse pointer is moved outside of the YouTube player window, playback stops. This is problematic because it makes it impossible to do something else whilst the video is playing in a window. If the window is maximised, obviously this isn't a problem. Also this seems to be no problem on a mobile platform.
Note that YouTube videos uploaded to other parts of the site, for example embedded in forum postings or articles, show none of this behaviour. It seems to be specific to CB gallery.
